1. Field of the Invention
The present invention relates to the measurement of jitter in a digital signal. In theory, the spacing of the transitions between levels of a digital signal have a completely uniform spacing. In practice, particularly during transmission, there may be minute variations in the actual time of the transition, relative to the theoretical transition time defined by an absolute reference clock. These variations are referred to as jitter, and may be considered to be a spurious phase modulation of the signal.
2. Summary of the Prior Art
Known systems for measuring jitter involve a very stable phase-locked loop which compares the pulse train containing jitter with an internally generated, jitter-free reference clock. The phase-locked loop has a generator for generating the reference clock, the output of which is fed to the input of a phase demodulator which also receives a digital signal containing jitter. The phase demodulator converts the signal to pulse duration modulation, which is output to a low pass filter, the output of which gives the jitter measurement, and also is fed back to the input of the reference clock generator, to form the loop. The low pass filter has cut off frequency of 5-10% of the bit rate. But since the digital signal being investigated may contain long sequences of digital zeros, a pattern/clock converter may be used to convert the digital signal into a continuous pulse train with the same jitter as the original signal, which pulse train then forms the input to the phase demodulator. Analysis of the output may involve peak value rectification before the results are displayed, and/or analysis with a spectrum analyser.
As mentioned above, such a jitter measurement system involves a low pass filter, and this has a significant influence on the greatest measurable jitter frequency component. The known systems also involve many analog circuits, which are more expensive than digital components.